Manufacturers and those who utilize information handling systems have become interested in utilizing optical fibers for transmitting signals. Optical fibers include a round inner glass core coated with a material having a different index of refraction from that of the core. Light is transmitted along the core and reflected internally by the coating. Optical fibers may be enclosed in a protective sheath either as a single transmission line or as a bundle of fibers forming an optical cable. A single optical fiber has the potential to provide simultaneous bidirectional communication, however, as used in information systems today optical fibers are usually connected between optical sub-assemblies which either transmit or receive optical signals. Examples of various means for providing connections between optical fibers and electronic circuitry are illustrated in U.S. Pat. Nos. 4,273,413 (Bendiksen et al), 4,547,039 (Caron et al), 4,647,148 (Katagiri), 4,707,067 (Haberland et al. ) and 5,005,939 (Arvanitakis et al. ) which are all incorporated herein by reference.
Optical modules include a two-part housing as described in U.S. Pat. No. 5,005,939 (Arvanitakis et al.). The housing provides two receptacle sections for mounting one or more and most commonly, two barrel-shaped optical sub-assemblies. Typically, one optical sub-assembly is a light transmitter for converting an electrical signal into an optical signal and the other is a light receiver for converting the optical signal into an electrical signal. The housing provides for precise alignment of the optical sub-assemblies with optical fibers contained in a suitable plug-in connector. Also within the housing is an electrical interconnect structure, typically a ceramic substrate with a circuit of screen printed electrical conductors on the upper surface, with electronic circuits connected to the electrical circuit on the upper surface. The internal interconnect structure includes leads or pins which protrude through apertures out of the housing to connect to an external electrical interconnect structure, typically a printed circuit board, to complete the optical-electrical connection.
One end of each optical sub-assembly communicates with a respective optic cable and from the other end, conductive pins extend axially for electrical connection to an adjacent edge of the internal interconnect structure in the housing. The central axis of each barrel-shaped optical sub-assembly extends parallel to the planer internal interconnect structure. The pins extend from the adjacent ends of the optical sub-assembly substantially above the interconnect structure so the pins are bent into an elbow or S-shape for soldered or welded connection to interconnection pads on the internal interconnect structure which provides electrical connection to the electronic circuit.
The interconnection pads on the internal interconnect structure are made by producing a solder pad on top of a conductive pad of the electrical circuit on the top surface of the ceramic substrate.
Recently, in U.S. Pat. No. 5,005,939 (Arvanitakis), it was proposed as an alternative to such soldering of the optical subassembly pins directly to the interconnection pads of the internal interconnect structure, that a flexible interconnect structure be used to connect between the pins and the interconnection pads. That patent discloses one end of a ribbon cable soldered to the pins of an optical sub-assembly and a distal end of the cable soldered to the interconnection pads of the internal interconnect structure. That patent also disclosed that utilizing a flexible ribbon cable would reduce electromagnetic interference and that additional ESD/EMI protection could be provided by providing a multilayer ribbon cable which included a ground layer.
Materials and processes for manufacturing conventional flexible ribbon cables are well known, for example, U.S. Pat. No. 4,906,803 (Albrechta) and U.S. Pat. No. 4,435,740 (Huckabee et al. ), incorporated herein by reference, describe production of a flexible cable including a conductive circuit layer which may be copper and a dielectric layer of polymer such as Kapton.RTM.. Typically a conductive metal film is coated with a positive or negative photoresist which is exposed to electromagnetic radiation using a mask and cured and otherwise processed to produce a photoresist pattern. The metal film which is not covered by the photoresist is selectively chemically etched to form the conductive circuit layer. The photoresist is then usually removed. Dielectric layers are etched using a similar chemical process or by laser etching/ablation to form windows through the dielectric layer. The dielectric is laminated onto one or both sides of the circuit layer with the windows positioned for interconnection of the cable to pins of electronic components and termination connections to pads on electrical interconnect structures.
The terminal connections to pads are formed by laying the exposed copper conductive paths or leads across conductive pads on the substrate and welding or soldering. U.S. Pat. No. 4,697,061 (Sparer et al.) describes a process in which a tin coated copper covering is ND-YAG laser welded to a tin coated copper base which is soldered to a screen printed circuit on top of a ceramic substrate. A hold-down clamp presses the cover against the base during welding. U.S. Pat. No. 4,906,812 (Nied et al.) describes a machine for laser welding, braising or soldering in an inert atmosphere. U.S. Pat. No. 4,825,034 (Auvert et al.) discusses a laser machine with movable platen and U.S. Pat. No. 5,048,034 (Tulip) disclose a Nd:YAG laser machine. U.S. Pat. No. 4,926,022 (Freedman) describes a laser soldering procedure in which a continuous wave Nd:YAG laser beam is directed only at the pad while a hold-down appliance presses the lead into the pad. The patent also describes problems encountered in attempting to heat the lead for such soldering including non co-planarity, overheating, and obscuring the lead with the hold down appliance. U.S. Pat. No. 5,021,630 (Benko et al.) discloses utilizing clear glass for holding down the lead during laser soldering. Finally, U.S. Pat. No. 5,008,512 (Spletter) discloses a copper lead coated with tin or indium and laser soldered to a gold electrical bump with a pulsed YAG laser.